Unanswered: How Combobox chain event

Unanswered: How Combobox chain event

I have a combobox, when the value change (select event) it will reload the store of other combobox, however the other combobox will always close by itself when you select the dropdown. This is certainly weird behavior of poor library created, I really wish i can use other product instead of ext js, however my boss decide to buy he because it is has relationship with your boss.

Please answer my question before asking what relationship they have.
From the large amount of javascript libraries which is reliable, they choose ext js which is unreliable, buggy and worse of all, it has so many unanswered question in the forum, which the dev itself might not know the answer